calling a protected virtual method in C++

Question

Possible Duplicate:
Calling virtual method in base class constructor
Calling virtual functions inside constructors

How can I call a protected virtual method from a constructor in C++?

class Foo
{
   Foo(){
       printStuff();  // have also tried this->printStuff()
   }
  protected:
   virtual void printStuff() {}
}

class ExtendedFoo : public Foo {
  protected:
   virtual void printStuff() { cout << "Stuff" << endl;}
}

...

ExtendedFoo exFoo; // should print "Stuff"

see also http://stackoverflow.com/questions/962132/calling-virtual-functions-inside-constructors — D.Shawley, Mar 30 '12 at 14:31

score 3 · Accepted Answer · answered Mar 30 '12 at 14:33

3

There's no problem in calling a protected function from the constructor - just do it. However, what you seem to be wanting is to call into a concrete derived class' implementation of it, e.g., ExtendedFoo's, since it's virtual - right? That's a no-go, since inside the Foo constructor, the object being created is still of type Foo, not ExtendedFoo, so no virtual dispatch can take place. If the protected function isn't pure virtual, the Foo implementation is called, i.e., the constructor will call the class' own implementation.

answered Mar 30 '12 at 14:33

Johann Gerell

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According to the standards, call to a pure virtual member inside a constructor is undefined behaviour. – j4x Mar 30 '12 at 16:16
Yes, that's why I wrote that a call to a **non-pure** virtual function will "succeed". – Johann Gerell Mar 30 '12 at 20:05

score 1 · Answer 2 · answered Mar 30 '12 at 14:44

Consider that when your base constructor is called, your actual constructor still does not, so you object is not completely formed.

If your object isn't yet formed, you can't expect it to act correctly.

Please read this:

score 0 · Answer 3 · edited May 23 '17 at 11:49

0

You can, but you will get Foo's implementation because ExtendedFoo's not been constructed. This is defined.

Similar problem: C++ design pattern: multiple ways to load file

edited May 23 '17 at 11:49

Community

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answered Mar 30 '12 at 14:30

justin

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score 0 · Answer 4 · answered Mar 30 '12 at 14:32

0

Answer deprecated after question change:

If it is protected in ExtendedFoo, you can not call it from outside of ExtendedFoo. The line...

exFoo.printStuff();

violated the function's protection level.

http://publib.boulder.ibm.com/infocenter/lnxpcomp/v8v101/index.jsp?topic=%2Fcom.ibm.xlcpp8l.doc%2Flanguage%2Fref%2Fcplr129.htm

answered Mar 30 '12 at 14:32

Foggzie

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thanks you're right, i phrased my question badly, corrected that – chillitom Mar 30 '12 at 14:33

calling a protected virtual method in C++

4 Answers4